Nuclear medicine is a field of medicine concerned with the use of radiation for diagnostic purposes. Single Photon Emission Computed Tomography (referred to in this specification as “SPECT”), a branch of nuclear medicine, involves directly measuring gamma rays emitted by radionuclides administered to a patient to produce slice-like images of the patient. “Tomography” refers to the production of slice-like images, or tomograms. Computerized Tomography (CT) refers to the use of computer processing to derive the tomogram.
Typically, in SPECT procedures, radiopharmaceuticals (otherwise known as radioactive tracers or radiotracers) are administered to patients. Radiopharmaceuticals are generally compounds consisting of radionuclides (i.e. radiation-emitting atoms), combined with pharmaceuticals or other chemical compounds. In some cases, such as with Thallium-201, the same particle is simultaneously the radionuclide and pharmaceutical. Unlike positron emission tomography (PET), which uses small radionuclides with half-lives of just over a minute to under 2 hours, SPECT involves the use of radionuclides whose half-life is several hours to days long, long enough to clinically localize or become fixed in specific organs or cellular receptors. In these circumstances, it is possible to acquire important diagnostic information by obtaining images created from the radiation emitted by the radiopharmaceutical.
One frequently-performed SPECT diagnostic procedure is myocardial perfusion imaging (MPI). Approximately ten million such scans are performed in the U.S. annually. For MPI, the patient is injected with a radioactive tracer which collects in, and becomes fixed in, the heart muscle. The localization of the tracer within the heart is dictated by the blood flow through the coronary arteries. These are the blood vessels that supply blood directly to the heart muscle, the myocardium. Therefore, MPI provides important information about the blood flow through the coronary arteries to the heart muscle. Thus, MPI can be used to diagnose serious and potentially fatal heart conditions such as coronary artery disease.
In prior art configurations, the time used to complete a SPECT scan is typically fixed. The camera being used is typically configured to operate for a predetermined period of time receiving photons from the patient. This is the case, for example, with conventional Anger-type cameras, as well as the GE™ 530c, a newer CZT camera. The Spectrum Dynamics D-SPECT™ camera uses a pre-scan performed prior to the diagnostic scan to determine the length of the diagnostic scan. Thus, scans done according to both these methods (most often the former) can be unduly long, causing undesirable delay both systemically and to individual patients.